To solve this question, the user needs to understand the difference between functional and non-functional testing. Functional tests evaluate the behavior of a system or component against the specifications, while non-functional tests evaluate the non-behavioral aspects of a system or component such as its performance, reliability, and usability.

Now, let's go through each option and determine which ones contain only non-functional tests:

A. Interoperability (compatibility) testing, reliability testing, performance testing: This option contains both functional and non-functional tests. Interoperability and compatibility testing is a type of functional testing, while reliability and performance testing are non-functional tests.

B. System testing, performance testing: This option contains both functional and non-functional tests. System testing is a type of functional testing, while performance testing is a non-functional test.

C. Load testing, stress testing, component testing, portability testing: This option contains only non-functional tests. Load testing and stress testing are non-functional tests that evaluate the performance of a system under different loads and stress levels. Component testing is a type of functional testing that evaluates the behavior of individual components, but portability testing is a non-functional test that evaluates the ability of a system to run on different hardware and software platforms.

D. Testing various configurations, beta testing, load testing: This option contains both functional and non-functional tests. Testing various configurations and beta testing are types of functional testing, while load testing is a non-functional test.

Therefore, the answer is: C. Load testing, stress testing, component testing, portability testing.